In recent years, the communication field has achieved notable technical progress in reducing the size and weight of a communication device such as a portable telephone. To achieve such technical progress, simplified and miniaturized components with various functions have been developed. At present, it is critical to improve the performance of the resonator and the filter used in communication devices.
The existing SAW (Surface Acoustic Wave) resonator has drawbacks because it has a low power handling capacity and is difficult to process when used for a high frequency situation.
The existing SAW resonator is characterized by good bandwidth selectivity (or “squareness index”), low insertion loss and small dimensions. However, as the existing SAW resonator has low power handling capacities, its application is limited when higher power handling capacities are necessary. The existing dielectric filter, on the other hand, has characteristics of low insertion loss and high power handling capacities, but has poor bandwidth selectivity (or “squareness index”) and large dimensions so that the usefulness of the existing dielectric filter is rather limited when high selectivity is required. If its selectivity is to be increased, the insertion loss and dimension will be increased as well, which in turn increases the cost of the application.
Chinese Patent No. 97106837.2, entitled High-Frequency Device with Distributed Electric Domain Ferroelectric Crystal Acoustic Superlattice disclosed a ferroelectric multilayered-film acoustic superlattice crystal material, referred to as a dielectric acoustic superlattice crystal material, which is a microstructure dielectric whose piezoelectric coefficient is periodically modulated, with a modulation period similar to the wavelength of an ultrasonic wave (i.e., within micron or submicron ranges). The superlattices are formed along a certain direction with alternately arranged positive and negative ferroelectric domains, where the piezoelectric coefficient alternately changes signs “+” and “−” corresponding to positive domains and negative domains. As illustrated in FIGS. 1 and 2, when an external alternating electric field is applied, the domain interface vibrates and forms elastic waves that are propagated in the ferroelectric domains. In the figures, arrows represent propagation directions of the elastic waves, and the grids represent directions of the electric domains. Further, the modes of plating electrodes are different in FIGS. 1 and 2 (where the black side represents the side where an electrode is plated). Specifically, the side where an electrode is plated is perpendicular to the direction of the electric domain in FIG. 1 while it is parallel thereto in FIG. 2. According to different modes of plating electrodes, there are a total of two vibration modes. FIG. 1 illustrates a vibration mode that the sound propagation direction is perpendicular to the electric field direction, and FIG. 2 illustrates a vibration mode that the sound propagation direction is parallel to the electric field direction. When a wave vector of the elastic wave is equal to a superlattice modulated wave vector, a resonance enhancement effect is produced, showing characteristics of a resonator and the resonance frequency is determined only by the period of the superlattice.